High-electron-mobility transistors (HEMTs), also known as heterostructure field effect transistors (HFETs), are field-effect transistors that include a heterojunction that acts as the transistor channel. In HEMTs, conduction of a “two-dimensional electron gas” in the heterojunction channel is regulated by a gate.
Despite invention in the late 1970's and the commercial success of HEMTs in applications such as, e.g., millimeter wave switching, commercial development of some HEMTs, e.g., gallium nitride-based HEMTs for power electronics has been slower than desired.
Field plates are conductive elements that have commonly been used to modify the profile of electric fields in semiconductor devices. In general, field plates are designed to reduce the peak values of electric fields in semiconductor devices, hence improving the breakdown voltages and lifespans of the devices that include field plates.
In HEMTs (e.g., gallium nitride-based HEMTs), it is believed that field plates also reduce a parasitic effect commonly referred to as “dc-to-rf dispersion” or “drain current-collapse.” During relatively higher frequency (e.g., radio frequency) operation, devices suffering from this parasitic effect reach lower drain current levels than the drain current levels reached during direct current (dc) operation. The parasitic effect is believed to be due to the relatively slow response times of interfacial states.
Experimental investigations into the length of field plates in HEMTs have been conducted. For example, researchers have described that, in some HEMT devices, breakdown voltage approaches a maximum value (i.e., “saturates”) after gate-connected field plates extend a certain distance toward the drain. Further extension of gate-connected field plates beyond the saturation length toward the drain yields little or no improvement in breakdown voltage. Since the input capacitance of the gate increases with proximity of a gate-connected field plate to the drain, it has been recommended that the extension of gate-connected field plates toward drains be limited once the saturation length is reached.